Many relatively large turbine engines, including aircraft main engines, may use an air turbine starter to initiate turbine engine rotation. The air turbine starter may be coupled to a high pressure fluid source, such as compressed air, which impinges upon a turbine in the air turbine starter causing it to rotate at a relatively high rate of speed. The air turbine starter includes an output shaft that is coupled to the turbine and, perhaps via one or more gears, to the main engine gearbox such that the main engine is rotated with the air turbine starter. When the main engine reaches a predetermined speed, the main engine is ignited such that a self-sustaining speed is achieved and operation is enabled without further assistance. More recently developed main engines have tighter starting tolerances, thereby requiring a more precise control of the start operation. As an example, the main engine may require a cold motoring operation in which the main engine is rotated for a predetermined amount of time prior to ignition.
Typically, the pneumatic power provided to start the main engine via the air turbine starter is provided by an auxiliary power unit (APU). Generally, such APUs may include one or more compressors, combustors, and turbines to generate mechanical and electrical power, particularly when the aircraft is on the ground. Additionally, air from one of the APU compressors may be used to drive the main engine via the air turbine starter. Although conventional APUs have been used to provide satisfactory assistance during start operations of aircraft main engines, more efficient uses of the APUs are desired, particularly with respect to controlling the speed of the main engine during start operations.
Accordingly, it is desirable to provide improved systems and methods for controlling aircraft main engines and the APU while the APU is providing the pneumatic energy for starting the aircraft main engines. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.